Oscillator circuit for receiving a wide frequency band signal

ABSTRACT

An oscillator provided with an oscillation circuit provided with two oscillation transistors comprising a differential pair and a resonance circuit connected in common to the bases of the oscillation transistors. The bases of the oscillation transistors are short-circuited by a coil, a center tap coil is connected in parallel to the resonance circuit, and variable capacitive diodes in the resonance circuit are driven by a coil connected to a mixing circuit. Due to this, it is possible to prevent the occurrence of low frequency noise, possible to realize a completely balanced operation, possible to avoid the oscillation carrier flowing into the power source and ground, and possible to reduce the noise in a television picture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oscillator used for a televisiontuner etc. for receiving a wide frequency band signal.

2. Description of the Related Art

FIG. 1 is a circuit diagram showing the basic configuration of this typeof oscillator as a related art.

In FIG. 1, OSC represents an oscillation circuit, RSN a resonancecircuit, MIX a mixing circuit, and AMP_(DC) a DC amplifier.

The oscillation circuit OSC is integrated and is comprised of npn typetransistors Q₁ and Q₂ comprising a differential type Colpitz oscillationcircuit, npn type transistors Q₃ and Q₄ comprising a differential outputstage, a biasing constant voltage source V₁ of the oscillation npn typetransistors Q₁ and Q₂, resistance elements R₁ to R₄, and constantcurrent sources I₁ to I₃.

The base of the oscillation transistor Q₁ is connected to aninput/output terminal T₁, is connected through the resistance element R₁to a line of a constant voltage source V₁, and is connected through theresistance element R₃ to the base of the transistor Q₃. The emitter ofthe oscillation transistor Q₁ is connected to an input/output terminalT₂ and is connected to the constant current source I_(I), which constantcurrent source I₁ is grounded. The collector of the oscillationtransistor Q₁ is connected to a line of a power source voltage V_(cc).

The base of the oscillation transistor Q₂ is connected to aninput/output terminal T₄, is connected through the resistance element R₂to the line of the constant voltage source V₁, and is connected throughthe resistance element R₄ to the base of the transistor Q₄. The emitterof the oscillation transistor Q₂ is connected to an input/outputterminal T₃ and is connected to the constant current source I₃, whichconstant current source I₂ is grounded. The collector of the oscillationtransistor Q₂ is connected to the line of the power source voltageV_(cc).

The transistors Q₃ and Q₄ are connected at their emitters and a node ofthe two is connected to the constant current source I₃. The constantcurrent source is grounded. The collectors of the transistors Q₃ and Q₄are connected to the input of the mixing circuit MIX.

Further, the output terminal of the mixing circuit MIX is connectedthrough the load resistance elements R₅ and R₆ to the line of the powersource voltage V_(cc) and is connected to the input of the DC amplifierAMP_(DC).

The resonance circuit RSN is comprised of a series circuit of a variablecapacitive diode VC₁ and coil L₁ to which are connected in parallel thecapacitors C₁ and C₂.

The node between one end of the coil L₁ of the resonance circuit RSN andthe capacitors C₁ and C₂ is grounded through the resistance element R₇of the resistance value of 30 kΩ and is connected to the input/outputterminal T₁ (base of oscillation transistor Q₁) through the DC cuttingcapacitor C₃.

The node between the cathode of the variable capacity diode VC₁ and thecapacitors C₁ and C₂ is connected to the output of the DC amplifierAMP_(DC) through a drive resistance element R₈ of a resistance value of30 kΩ and is connected to an input/output terminal T₄ (base ofoscillation transistor Q₂) through a DC cutting capacitor C₃.

Between the node of the capacitor C₃ and the input/output terminal T₁and the input/output terminal T₂ is connected a positive feedbackcapacitor C₅ and between the node of the capacitor C₄ and theinput/output terminal T₄ is connected a positive feedback capacitor C₆.

Further, between the node of the capacitor C₅ and the input/outputterminal T₂ and the node of the capacitor C₆ and the input/outputterminal T₃ (between the emitter of the oscillation transistor Q₁ andthe emitter of the oscillation transistor Q₂) is connected a couplingcapacitor C₇.

Note that the capacities of the externally provided capacitors C₁ to C₇are set for example as follows: 1 pF for the capacitor C₁, 13 pF for thecapacitor C₂, 56 pF for the capacitors C₃ and C₄, 2 pF for thecapacitors C₅ and C₆ and 3 pF for the capacitor C₇.

In such a configuration, the oscillation circuit OSC receives positivefeedback from the capacitors C₅ and C₆ connected between the bases andemitters of the oscillation transistors Q₁ and Q₂ through theinput/output terminals T₁, T₂, T₃, and T₄, oscillates at the resonancefrequency of the resonance circuit RSN connected to the bases of theoscillation transistors Q₁ and Q₂, and outputs the local oscillationfrequency signal S_(L) of a predetermined frequency through thetransistors Q₃ and Q₄ to the mixing circuit MIX.

Note that the oscillation transistors Q₁ and Q₂ comprising thedifferential Colpitz oscillation circuit are connected at their basesthrough the resonance circuit RSN, so perform oscillation operationswith opposite phases.

Accordingly, local oscillation frequency signals S_(L) with oppositephases are output from the collectors of the transistors Q₃ and Q₄.

In the mixing circuit MIX, the FM modulated video signal of the selectedchannel and the local oscillation frequency signal S_(L) are mixed, andthe signal of the frequency of the difference is taken out and is outputto the DC amplifier AMP_(DC). Further, the resonance circuit RSNa isdriven through the coil L₄ by the output of the DC amplifier AMP_(DC).

That is, after the phase detection by the mixing circuit MIX, thecontrol voltage of the demodulation frequency, which is based on thedrive resistance element R₈ and the output of the DC amplifier AMP_(DC),is supplied to the cathode of the variable capacity diode VC₁ of theresonance circuit RSN.

However, the above-mentioned circuit is used as an oscillator for an FMdemodulator for satellite broadcasting (BS) and the frequency is 400 MHzto 500 MHz.

However, in the above-mentioned circuit, it suffers from thedisadvantage that it is not possible to obtain a satisfactory picturesince noise of a low frequency band would occur, which could be readilydiscerned even by the human eye, in the television picture, due to theso-called shot noise, flicker noise, or burst noise arising from theconstruction of the junction portion of the oscillation transistors Q₁and Q₂ itself and the lattice faults, hot noise due to the resistanceelements R₁ and R₂, etc.

Further, the control voltage of the demodulation frequency based on theoutput of the drive resistance element R₈ and the DC amplifier AMP_(DC)is supplied to the cathode of the variable capacity diode VC₁ of theresonance circuit RSN, but the resistance value of the drive resistanceelement R₈ is, as explained above, 30 kΩ, the time constant with thecapacitors is large, the drive impedance of the variable capacity diodeVC₁ is high, the frequency characteristics will not improved, andtherefore this becomes a cause behind low frequency noise.

Since the operational frequency is a high of about 400 MHz to 500 MHz orso, the oscillation carrier easily flows into the power source andground and therefore it suffers from the disadvantage that there is aliability of occurrence of a pseudo lock and a beat by the highfrequency component.

SUMMARY OF THE INVENTION

The present invention was made in consideration of these circumstancesand has as its object to provide an oscillator which reduces noise,operates stably without the oscillation carrier flowing to a powersource or ground, can prevent the occurrence of a pseudo lock and highfrequency beat even at a weak electric field, and therefore can improvethe quality of a television picture.

According to the present invention, there is provided an oscillatorcomprising: a differential type amplifier formed by first and secondtransistors; a resonance circuit, output terminals thereof is connectedto bases of the first and second transistors; and a first inductiveelement connected between the bases of the first and second transistors.

According to the present invention, the bases of the first and secondtransistors are connected by the first inductive element, so theimpedance across the bases becomes low at the time of the oscillationoperation and therefore the so-called shot noise, flicker noise, burstnoise, etc. arising due to the construction of the junction portion ofthe first and second transistors themselves or lattice faults arecanceled out and the occurrence of low frequency noise at the televisionpicture can be suppressed.

Preferably, the oscillator further includes a second inductive elementconnected to the resonance circuit in parallel, a point dividedinductance thereof into substantially equal two values is grounded.

According to the present invention, the oscillation circuit becomes oneof a completely balanced configuration due to the second inductiveelement. Accordingly, a stable oscillation operation is achieved withoutthe oscillation carrier flowing into the power source or ground, wherebythe occurrence of a pseudo lock or a high frequency beat can beprevented even with a weak electric field.

Also, preferably, the oscillator further includes a mixing circuithaving input terminals connected to output terminals of the amplifier,and a third inductance element connected between an output terminal ofthe mixing circuit and a frequency control terminal of the resonancecircuit.

The resonance circuit is driven by a signal which is phase-detected inresponse to an output signal of the amplifier in the mixing circuit andis applied to the frequency control terminal through the third inductiveelement.

According to the present invention, the resonance circuit is driven by alow impedance through the third inductive element at a demodulationfrequency. Due to this, the frequency characteristics are improved andthe occurrence of low frequency noise is suppressed.

Preferably, the resonance circuit includes at least first and secondvariable capacitive elements oppositely connected in series. Thefrequency control terminal is provided at a point commonly connected thevariable capacitive elements, to vary the capacitances of the first andsecond variable capacitive elements in response to a signal from thesecond inductive element.

Also, preferably, the oscillator further includes a first capacitiveelement connected between the base and emitter of the first transistorof the amplifier, a second capacitive element connected between the baseand emitter of the second transistor of the amplifier, and a thirdcapacitive element connected between the emitters of the first andsecond transistors.

As the base and emitter of the first transistor are connected by thefirst capacitive element, the emitter of the first transistor and theground are connected by the second capacitive element, the base of thesecond transistor and the ground are connected by the third capacitiveelement, and the collector of the first transistor is connected to theground, the impedance of the second capacitive element and the firstinductive element itself can contribute to the oscillation. Further, thegrounding points become the third capacitive element and the collectorof the first transistor, so there is no effect from the unbalance of thedifferential pair due to variations in the elements or external noise,and the oscillation operation is controlled by just the peripheralelements of the first transistor at all times.

Alternatively, the oscillator further includes a first capacitiveelement connected between the base and emitter of the first transistor,a second capacitance element connected between the emitter of the firsttransistor and ground, and a third capacitive element connected betweenthe base of the second transistor and ground. A collector of the firsttransistor is connected to ground.

Preferably, the first inductance element has a point where inductancethereof is divided into substantially equal two values and is connectedto ground.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clearer from the following description of preferred embodimentswith reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram showing an example of the configuration ofan oscillator of the related art;

FIG. 2 is a circuit diagram showing a first embodiment of the oscillatoraccording to the present invention;

FIG. 3 is a circuit diagram showing a second embodiment of an oscillatoraccording to the present invention; and

FIG. 4 is a circuit diagram showing a third embodiment of an oscillatoraccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described.

First Embodiment

FIG. 2 is a circuit diagram showing a first embodiment of an oscillatoraccording to the present invention, wherein components the same as thosein FIG. 1 showing the related art are shown with the same references.

That is, OSC is an oscillation circuit, RSNa a resonance circuit, MIX amixing circuit, AMP_(DC) a DC amplifier, L₂ an oscillation coil, L₃ acenter tap coil, L₄ a drive/high frequency signal blocking coil, C₅ andC₅ positive feedback capacitors, and C₇ a coupling capacitor.

This circuit may be preferably used as an oscillator for an FMdemodulator for satellite broadcasting (BS) and operates at a frequencyof about 400 MHz to 500 MHz.

The oscillation circuit OSC is integrated and is comprised of npn typetransistors Q₁ and Q₂ comprising a differential type Colpitz oscillationcircuit, npn type transistors Q₃ and Q₄ comprising a differential outputstage, a biasing constant voltage source V₁ of the oscillation npn typetransistors Q₁ and Q₂, resistance elements R₁ to R₄, and constantcurrent sources I₁ to I₃.

The base of the oscillation transistor Q₁ is connected to theinput/output terminal T₁, is connected through the resistance element R₁to the constant voltage source V₁, and is connected through theresistance element R₃ to the base of the transistor Q₃. The emitter ofthe oscillation transistor Q₁ is connected to the input/output terminalT₂ and is connected to the constant current source I₁, which constantcurrent source I₁ is grounded. The collector of the oscillationtransistor Q₂ is connected to the line of the power source voltageV_(cc).

The base of the oscillation transistor Q₂ is connected to theinput/output terminal T₄, is connected through the resistance element R₂to the constant voltage source V₁, and is connected through theresistance element R₄ to the base of the transistor Q₄. The emitter ofthe oscillation transistor Q₂ is connected to the input/output terminalT₃ and is connected to the constant current source I₂. The constantcurrent source I₂ is grounded. The collector of the oscillationtransistor Q₂ is connected to the line of the power source voltageV_(cc).

Further, the transistors Q₃ and Q₄ are connected at their emitters, thenode between the two is connected to the constant current source I₃, andthe constant current source I₃ is grounded. Further, the collectors ofthe transistors Q₃ and Q₄ are connected to the input of the mixingcircuit MIX.

Further, the output terminal of the mixing circuit MIX is connectedthrough the load resistance elements R₅ and R₆ to the line of the powersource voltage V_(cc) and connected to the input of the DC amplifierAMP_(DC).

The resonance circuit RSNa is comprised primarily of the two variablecapacity diodes VC₁ and VC₂ with cathodes connected. The oscillationcoil L₂ and the center tap coil L₃ are connected in parallel to thesevariable capacity diodes VC₁ and VC₂, while the drive/high frequencysignal blocking coil L₄ is connected between the node between thecathodes of the two variable capacity diodes VC₁ and VC₂ and the outputof the DC amplifier AMP_(DC).

As the oscillation coil L₂, use is made of a coil comprised of forexample 2 to 4 turns (T) and of about 20 Ω at a frequency of 400 MHz.One end of the oscillation coil L₂ is connected to the node of theinput/output terminal T₁ and the positive feedback capacitor C₅, whilethe other end is connected to the node between the input/output terminalT₄ and the positive feedback capacitor C₆. That is, due to theoscillation coil L₂, the bases of the oscillation transistors Q₁ and Q₂are connected by a low impedance. Further, the oscillation frequency isdetermined based on the oscillation coil L₂ and the variable capacitydiodes VC₁ and VC₂. The oscillation frequency f is given by thefollowing formula:

    f=1/{2R(L·CD/2).sup.1/2 }                         (1)

The capacities of the variable capacity diodes VC₁ and VC₂ are set toabout 10 to 20 pF each. The anode of the variable capacity diode VC₁ isconnected through the coupling capacitor C₈ to the node between one endof the oscillation coil L₂ and the positive feedback capacitor C₅. Theanode of the variable capacity diode VC₂ is connected through thecoupling capacitor C₉ to the node between the other end of theoscillation coil L₂ and the positive feedback capacitor C₆.

The capacities of the coupling capacitors C₈ and C₉ are set to forexample 30 to 50 pF.

The center tap coil L₃ used is a coil comprised of for example 10 T andof about 200 Ω at a frequency of 400 MHz. That is, L₃ >>L₂.

One end of the center tap coil L₃ is connected to the anode of thevariable capacity diode VC₁, while the other end is connected to theanode of the variable capacity diode VC₂. Further, the center point(tap) is grounded. Due to this, the variable capacity diodes VC₁ and VC₂are held at the ground potential.

Note that the impedance of the center tap coil L₃ is sufficiently highwith respect to the oscillation frequency f, so there is no need for thecenter tap to be positioned strictly at the center point of the coil andmay be suitably positioned.

Further, as the center tap coil L₃, instead of grounding the centerpoint of one coil, it is also possible to connect two coils of the samenumber of turns in series and ground the node of the same.

The drive/high frequency signal blocking coil L₄ used is a coil which isfor example comprised of 15 T and has a high impedance at theoscillation frequency and a low impedance, for example, several tens ofohms, at the demodulation frequency. Further, at the low frequency, forexample, less than 100 kHz, it is held to less than 0.063 Ω.

The drive/high frequency signal blocking coil L₄ is connected to thenode of the cathodes of the variable capacity diodes VC₁ and VC₂ at oneend and is connected to the output of the DC amplifier AMP_(DC) at theother end.

Next, an explanation will be made of the operation according to theabove configuration.

The oscillation circuit OSC functions as a so-called completely balancedtype Colpitz oscillation circuit since the center point of theexternally provided center tap coil L₃ is grounded.

Such a completely balanced type oscillation circuit OSC receives, in theinductive region of the so-called parallel tank circuit between theoscillation coil L₂ and the variable capacity diodes VC₁ and VC₂,positive feedback by the capacitors C₅ and C₆ connected between thebases and emitters of the oscillation transistors Q₁ and Q₂ through theinput/output terminals T₁ and T₂, oscillates at the resonance frequency,and outputs the local oscillation frequency signal S_(L) of thepredetermined frequency to the mixing circuit MIX through thetransistors Q₃ and Q₄.

Note that the oscillation transistors Q₁ and Q₂ comprising thedifferential Colpitz oscillation circuit are connected at their basesthrough the resonance circuit RSNa, so perform oscillation operations ofdifferent phases from each other.

Accordingly, local oscillation frequency signals S_(L) of oppositephases are output from the collectors of the transistors Q₃ and Q₄.

At this time, the bases of the oscillation transistors Q₁ and Q₂ areconnected by the coil L₂ of the low impedance near the oscillationfrequency, so the so-called shot noise, flicker noise, burst noise, etc.arising due to the construction of the junction portion of the first andsecond transistors Q₁ and Q₂ themselves, the hot noise due to theresistance elements R₁ and R₂, etc. are canceled out and the occurrenceof low frequency noise at the television picture can be suppressed.

In the mixing circuit MIX, the FM modulated video signal of the selectedchannel and the local oscillation frequency signal S_(L) are mixed, andthe signal of the frequency of the difference is taken out and is outputto the DC amplifier AMP_(DC). Further, the resonance circuit RSNa isdriven through the coil L₄ by the output of the DC amplifier AMP_(DC).

That is, after phase detection at the mixing circuit MIX, the controlvoltage of the demodulation frequency, which is based on the output ofthe coil L₄ and the DC amplifier AMP_(DC), is supplied to the cathode ofthe variable capacity diodes VC₁ and VC₂ of the resonance circuit RSNa.The oscillation circuit OSC finally oscillates locked to a frequencyaccording to the control voltage.

At this time, the coil L₄ is of a high impedance at the oscillationfrequency, but is a low impedance, for example, several tens of ohms, atthe demodulation frequency, so the frequency characteristics improvedand the occurrence of low frequency noise can be suppressed.

Further, since the oscillation circuit is configured for a completebalance, despite the fact the operational frequency is a high 400 MHz to500 MHz, it is possible to suppress the flow of oscillation carrier intothe power source and ground and therefore the occurrence of a pseudolock or a beat due to the high frequency component can be prevented.

As explained above, according to the present invention, the bases of theoscillation transistors Q₁ and Q₂ are connected by the low impedancecoil L₂, so the flicker and low frequency noise impedance becomes closeto zero and as a result the noise in the television picture can bereduced.

Further, since the center tap coil L₃ is provided, a completely balancedoperation can be realized in the oscillation circuit. As a result,stable operation can be realized without the oscillation carrier flowinginto the power source and the ground. Further, occurrence of a pseudolock and high frequency beat can be prevented even in a weak electricfield.

In addition, since the variable capacity diodes VC₁ and VC₂ are drivenby the coil L₄, the impedance of the flicker noise region can be keptlow and it is possible to suppress low frequency noise.

Second Embodiment

FIG. 3 is a circuit diagram showing a second embodiment of theoscillator according to the present invention.

The point of difference between the second embodiment and the firstembodiment is that instead of the local oscillation frequency signalS_(L) of the oscillation circuit OSC being output through the npn typetransistors Q₃ and Q₄, connected in parallel to the oscillationtransistors Q₁ and Q₂, from these transistors Q₃ and Q₄, the npn typetransistors Q₅ and Q₆ are connected in series between the oscillationtransistors Q₁ and Q₂ and the load resistance elements R₁₀ and R₁₁connected to the power source voltage V_(cc) and the local oscillationfrequency signal S_(L) is output from the nodes between the collectorsof the npn type transistors Q₅ and Q₆ and the load resistance elementsR₁₀ and R₁₁.

Further, between the emitters of the oscillation transistors Q₁ and Q₂and the ground are connected the resistance elements R₁₂ and R₁₃functioning as current sources.

The emitter of the transistor Q₅ is connected to the collector of theoscillation transistor Q₁, the emitter of the transistor Q₆ is connectedto the collector of the oscillation transistor Q₂, and the bases of thetwo transistors Q₅ and Q₆ are connected to the constant voltage sourceV₂.

Further, the constant voltage source V₁ is connected to the base of thepnp type transistor P₁, the emitter of the pnp type transistor P₁ isconnected to the node of the resistance elements R₁ and R₂, and thecollector is grounded.

In this configuration as well, a similar operation is performed as inthe first embodiment explained above. A similar effect to the effect ofthe first embodiment explained above is obtained, of course, and thereare the advantages that the current consumption can be reduced and theisolation with the mixing circuit MIX can be improved.

Third Embodiment

FIG. 4 is a circuit diagram of key parts showing a third embodiment ofthe oscillator according to the present invention.

The oscillation circuit according to this embodiment differs from thefirst embodiment and the second embodiment in that the transistors Q₁and Q₂ comprising the differential pair are not balanced and theoscillator is configured using just the transistor Q₂.

Therefore, an externally provided capacitor C₅ is connected between theinput/output terminal T₁ and the ground, a capacitor C7 is connectedbetween the input/output terminal T₃ and the ground, and the collectorsof the transistors Q₁ and Q₂ are grounded.

Further, instead of using the center tap coil L₃, the center point ofthe oscillation coil L₂ is grounded.

In such a configuration, the capacitor C7 and the coil L₂ contribute tothe impedance of the element itself.

Further, the grounding points become one of the electrodes of thecapacitor C₅ and the collector of the transistor Q₁, there is no effectfrom the imbalance of the differential pair due to variations in theelements or external noise, and the oscillation is controlled at alltimes by the peripheral elements of the transistor Q₂.

Note that in this embodiment too, like with the first embodimentexplained above, the bases of the oscillation transistors Q₁ and Q₂ areconnected by the coil L₂, so the flicker and low frequency noiseimpedance becomes close to zero and as a result the noise of thetelevision picture can be reduced. Further, since the variable capacitydiodes VC₁ and VC₂ are driven by the coil L₄, the impedance of theflicker noise region can be kept low and it is possible to suppress lowfrequency noise.

As explained above, according to the present invention, the flicker andthe low frequency noise impedance become close to zero and as a resultthe noise in a television picture can be reduced.

Further, since a so-called center tap inductive element is provided, theoscillation carrier will not flow into the power source and ground andtherefore a stable oscillation operation can be realized and it ispossible to prevent occurrence of a pseudo lock and high frequency beateven in a weak electric field.

What is claimed is:
 1. An oscillator comprising:a differential typeamplifier formed by first and second transistors; a resonance circuit,output terminals thereof being connected to bases of said first andsecond transistors; a first inductive element directly connected betweensaid bases of said first and second transistors; and a second inductiveelement connected to said resonance circuit in parallel, a pointdividing inductance thereof into substantially two equal values beinggrounded.
 2. An oscillator as set forth in claim 1, further comprising amixing circuit having input terminals connected to output terminals ofsaid differential type amplifier, and a third inductance elementconnected between an output terminal of said mixing circuit and afrequency control terminal of said resonance circuit,wherein saidresonance circuit is driven by a signal which is phase-detected inresponse to an output signal of said amplifier in said mixing circuitand is applied to said frequency control terminal through said thirdinductive element.
 3. An oscillator as set forth in claim 2, wherein afrequency control section is connected to said second inductive elementin parallel.
 4. An oscillator as set forth in claim 2, wherein saidresonance circuit comprises at least first and second variablecapacitive elements oppositely connected in series, andwherein saidfrequency control terminal is provided at a point commonly connectingsaid variable capacitive elements, to vary the capacitances of saidfirst and second variable capacitive elements in response to a signalfrom said second inductive element.
 5. An oscillator as set forth inclaim 1, further comprising a first capacitive element connected betweena base and an emitter of said first transistor of said differential typeamplifier,a second capacitive element connected between said base andemitter of said second transistor of said differential type amplifier;and a third capacitive element connected between respective emitters ofsaid first and second transistors.
 6. An oscillator as set forth inclaim 5, wherein said first and second capacitive elements function aspositive feedback capacitors, andwherein said third capacitive elementfunctions as a coupling capacitor.
 7. An oscillator as set forth inclaim 5, wherein said differential amplifier comprises a first currentsource connected between said emitter of said first transistor andground, and a second current source connected between said emitter ofsaid second transistor and ground.
 8. An oscillator as set forth inclaim 7, wherein said differential type amplifier comprises a voltagebiasing circuit connected to said bases of said first and secondtransistors.
 9. An oscillator as set forth in claim 5, wherein saidoscillator is formed as a differential Colpitz oscillation circuit. 10.An oscillator comprising:a differential type amplifier formed by firstand second transistors; a resonance circuit, output terminals thereofbeing connected to bases of said first and second transistors; a firstinductive element directly connected between said bases of said firstand second transistors; a first capacitive element connected between abase and an emitter of said first transistor; a second capacitanceelement connected between said emitter of said first transistor andground; a third capacitive element connected between a base of saidsecond transistor and ground; and a collector of said first transistorbeing connected to ground.
 11. An oscillator as set forth in claim 10,wherein said first inductance element has a point where inductancethereof is divided into substantially two equal values and that isconnected to ground.
 12. An oscillator as set forth in claim 11, furthercomprising a mixing circuit having input terminals connected to outputterminals of said amplifier, and a second inductive element connectedbetween an output terminal of said mixing circuit and a frequencycontrol terminal of said resonance circuit,wherein said resonancecircuit is driven by a signal which is phase-detected in response to anoutput signal of said amplifier in said mixing circuit and is applied tosaid frequency control terminal through said second inductive element.13. An oscillator as set forth in claim 12, wherein said resonancecircuit comprises at least first and second variable capacitive elementsoppositely connected in series, andwherein said frequency controlterminal is provided at a point commonly connecting said variablecapacitive elements, to vary the capacitances of said first and secondvariable capacitive elements in response to a signal from said secondinductive element.